(a) Field of the Invention
The invention relates to a mixing chamber for thickening of a sludge, especially of sludges in sewage clarification plants by means of a flocculation agent, (combined with overflow drainage means, whereby the mixing chamber has a mixing zone and a diversion for the thickened sludge leading to a sedimentation arrangement and/or a press filter.
(B) Description of the Prior Art
In the case of dehydration, drainage and purification of sludges, especially in the case of purification of sewage sludge, flocculation agents are added to the sludge in order to separate non-settlable suspended solids and colloidally dissolved substances under stirring forces within a mixing chamber. Since the suspended substances and colloidal particles in an aqueous environment have an equal electric charge on their surface, they themselves cannot coagulate into larger particles capable of easily sedimentation. In the case of the addition of flocculation agents which have a charge character opposite to the charge of the particles that are to be eliminated, the solid particles agglomerate with the flocculation agents while forming large flakes which are capable of easy sedimentation. The flocculation agents consist of natural or synthetic long, branched chain molecules, and especially synthetic molecules in the form of polymerizates used as the basis of the coagulating agent; namely, polyacrylamide, polyacrylate, polyethylene, imines and polyethyloxides have been most widely used.
In the case of the known sewage clarification plants, after the addition of flocculation agent, the thickened sludge is fed to the upper end of a revolving belt screen of the straining zone on which the flakes formed settle, while a large part of the water seeps through the screen belt consisting generally of a cloth and drops off. Generally, the water content of the sludge is reduced at the end of the stretch of the straining zone by about 60% so that then an aqueous sludge exists which can be subjected to a filtration process within a screen belt press.
In practice, it turns out that the thickened sludge must remain for a relatively long time in the straining zone in order to actually reach the degree of dehydration and drainage possible on such a screen belt of the straining zone. Because the first settled flakes will already occupy the holes of the screen belt or cloth whereupon the water from higher sludge and slime layers can seep through now only very slowly and drop off. For that reason, the screen belt for the straining zone must be made either very long or it must be allowed to run very slowly, which however leads to considerable installation and processing costs in both cases.